Well-known in the art are engines which are provided with a catalytic converter, such as three way catalyzer, within the exhaust system thereof. In these engines secondary so as to purify harmful contaminants, such as carbon monoxide (CO), hydrocarbon (HC) and nitrogen oxides (NO.sub.x), emitted from the engine. A sufficiently high temperature for causing the desirable activating reaction in the catalytic converter and an adequate air fuel ratio controlled so as to be maintained at the stoichiometric air fuel ratio are required for obtaining a high converting efficiency of the catalytic converter. (The term "secondary air fuel ratio" used hereinafter is defined as the total amount of air supplied to the engine, from the intake system to the exhaust system, to the amount of fuel.)
To maintain the secondary air fuel ratio at the stoichiometric air fuel ratio, a device for detecting the density of oxygen is disposed within the exhaust system at a position upstream of the catalytic converter, but downstream of the position where secondary air is supplied. A signal transmitted from the oxygen-detecting device is transferred to a computer which has an algebraic function therein for modulating the signal. The result obtained from the computer is utilized for stepwisely (ON-OFF) actuating a secondary air control valve device. As a result, secondary air is stepwisely supplied to the exhaust system of the engine.
The above-mentioned oxygen-detecting device can only detect whether the secondary air fuel ratio is on the lean side or the rich side of the stoichiometric air fuel ratio, and can not detect the variation of the secondary air fuel ratio from the stoichiometric air fuel ratio. Accordingly, when the secondary air is controlled stepwisely ON-OFF, in accordance with the signal transmitted from the oxygen-detecting device, the quantity of secondary air supplied may be more than or less than the amounts necessary to produce the stoichiometric air fuel ratio. As a result, the secondary air fuel ratio may be caused to fluctuate widely on both sides of the stoichiometric air fuel ratio. In addition, the operating characteristics of a carburetor and the secondary air control device may vary from the ideal characteristics due to different engine load conditions. This variation of the operating characteristics may increase the above-mentioned fluctuation of the secondary air fuel ratio.
To eliminate the above-mentioned problems, in some engines the secondary air flow flowing out of the secondary air control device has, when plotted on a graph with the amount of air as the ordinate and time as the abscissa, a special wave form, such as a triangular wave form, rather than a stepwise wave form. In these cases, the signal transmitted from the oxygen-detecting device is transferred to a computer having an algebraic function therein and the result obtained by the computer is utilized for actuating a plurality of actuating valve devices which are communicated with a vacuum source. Simultaneously, vacuum surge tanks, having small volumes and orifices, are utilized for transforming the wave form, obtained in a similar manner to that mentioned above, of the actuating vacuum transmitted from the actuating valves into a triangular wave form and for transmitting the triangular wave form actuating vacuum, to a single secondary air control device. The above-mentioned system, however, requires many complicated parts and connections for transforming the above mentioned wave form of the actuating vacuum signal of the secondary air control device into a triangular wave. Accordingly, the cost of the system is high, and the adjustment and maintenance of the system is troublesome. In addition, when the system is not adjusted or maintained properly, the reliability of the system can be lowered so that the required triangular wave form can not be obtained.